Occurrence of various types of penicillinase plasmid among 'hospital' staphylococci.

The type of penicillinase plasmid carried by penicillin-resistant cultures of Staphylococcus aureus isolated from eight London hospitals has been determined. Among ;endemic' hospital staphylococci, cultures resistant to mercury salts and producing large amounts of A-type penicillinase, a high proportion of which is extracellular, are most common. C-type penicillinase is rarely associated with resistance to mercury salts.The majority of strains resistant to penicillin and at least one other antibiotic carried the alpha-plasmid. Possible reasons for the prevalence of the alpha-plasmid among endemic hospital staphylococci are discussed.